Foamable polystyrene beads are relatively easy to make. In a typical method, polystyrene resin is impregnated with an expanding agent, usually pentane, during polymerization, or else resin particles are impregnated with the expanding agent after polymerization. These particles are then subjected to steam to partially expand them.
Foamable polystyrene beads are also easy to mold. In a typical method, the pre-expanded beads are fed into a mold and subjected to pressurized steam where they expand, fuse together, and conform to the shape of the mold. Such moldings are useful as decoration, insulation, and protective packaging.
However, expanded polystyrene moldings suffer from many disadvantages. Since polystyrene exhibits poor solvent resistance and is unstable at high temperatures, moldings made from polystyrene cannot be used for many applications. Furthermore, expanded polystyrene foam is generally brittle and fragile and possesses poor cushioning properties. These properties limit its use as protective packaging for fragile items such as computers and other delicate instrumentation. In addition, polystyrene foam does not stand up well to repeated impacts. In fact, the cushioning ability of the molding is usually severely impaired after just one impact.
The preparation of thermoplastic polymer foams by extruding a heat-plastified mixture of thermoplastic resin and a blowing agent is well known in the art and is described in U.S. Pat. Nos. 2,740,157, 3,067,147, 3,413,387, 3,413,388, 3,431,163, 3,431,164, 3,808,300, 3,954,929, 3,966,381, 4,640,933, 4,663,361, and 4,694,027, and in Canadian Patent No. 451,854, as well as in other literature pertaining to the art.
U.S. Pat. No. 2,450,436 discloses a method for the preparation of cellular thermoplastic polymer products. There, a solid thermoplastic resin, e.g., polystyrene, and a normally gaseous agent such as methyl chloride, methyl ether, propylene, or butylene are held in a closed vessel under pressure at a temperature below the critical temperature of the normally gaseous agent until a homogeneous mobile gel is obtained. Thereafter, an outlet is opened to permit flow of the gel from the vessel. During flow of the mobile gel from the pressurized vessel into a zone of lower pressure, the resin is swollen by vaporization and expansion of the dissolved volatile substance to form a stable cellular product consisting for the most part of individual closed thin-walled cells.
U.S. Pat. No. 2,515,250 describes a method of forming under pressure a mixture of predetermined proportions of a normally gaseous agent and a thermoplastic resin, and storing the mixture by feeding the same into a pressurized storage vessel in which it is maintained at a desired temperature until a homogeneous mobile gel or solution is obtained, prior to extrusion and expansion of the resin.
U.S. Pat. No. 3,067,147 discloses a method for the preparation of a cellular mass from thermoplastic resin by incorporating a gas or volatile organic liquid in the thermoplastic resin to be foamed. The mixture is heated to a temperature at which it becomes plastic and vapors of gas or volatile liquid expand the softened resin to form a cellular mass.
U.S. Pat. No. 2,387,730 teaches a method of making cellular polyethylene by impregnating a molten polymer with a gas which is soluble therein under pressure. The polymer is then expanded by partially releasing the pressure while maintaining the temperature, followed by cooling the expanded polymer.
U.S. Pat. No. 3,808,300 discloses a method for the preparation of closed cellular olefin polymers using a mixture of a citric acid salt, a carbonate or bicarbonate as the nucleating agent, and n-butane-isobutane mixtures for the foaming agent.
U.S. Pat. Nos. 4,640,933, 4,633,361 and 4,64,027 disclose methods for the preparation of expandable polyolefin compositions using isobutane and mixtures of isobutane, chlorofluorocarbons and fluorocarbons or a mixture of at least 70% isobutane and other hydrocarbons as the blowing agent for long chain fatty acids with polyols.
The preparation of thermoplastic foams containing either an antistatic agent or a flame retardant agent is well known in the art and is described in U.S. Pat. Nos. 4,298,710, 4,556,680, 4,626,563, 4,293,656, 4,286,071, 4,337,319 and 4,219,466.
U.S. Pat. No. 4,298,710 describes an antistatic resin composition of 100 parts of a nitrile copolymer and 0.05 to 10 parts of a surfactant added thereto as an antistatic additive. The nitrile copolymer comprises 7 to 100% of a nitrile graft formed by polymerizing a monomer mixture of a specific composition onto a rubber trunk polymer predominantly comprising a conjugated diolefin and/or an acrylate, and 0 to 93% of a nitrile random copolymer of a specific composition.
United States Defensive Publication T953,006 (Dec. 7, 1976) describes antistatic cellular polyolefin products and articles thereof. The cellular composition includes an antistatic agent, especially an amine having at least one long aliphatic hydrocarbyl chain or a salt thereof, especially a quaternary ammonium salt.
U.S. Pat. No. 4,626,563 discloses the preparation and use of flame retardant carbonate polymers containing an aromatic sulfimide, a monomeric or polymeric halogenated organic compound, a metal sulfate having a pka from 1 to 5 and a fibril forming polytetrafluoroethylene as additives in effective amounts giving carbonate polymers that not only are flame retardant, but are melt stable (i.e. show little loss in molecular weight during processing or melt shearing).
U.S. Pat. No. 4,293,656 describes a polystyrene foam combined with a halogen-containing flame retardant and 2,2 bis(4-allyloxy-3,5-dibromophenyl) propane, which is a synergist, present in a 0.01 to 1.0 weight percent based on the weight of polystyrene.
U.S. Pat. No. 4,286,071 and 4,337,319 teaches the use of bromine compounds and one synergist to make expandable styrene polymer flame retardant.
U.S. Pat. Nos. 4,219,071 and 4,337,319 teach the use of bromine compounds and one synergist to make expandable styrene polymer flame retardant.
U.S. Pat. No. 4,219,466 describes a resin composition having high impact resistance, improved release property, and reduced flammability by mixing a polymer containing a major amount of monovinyl aromatic monomer, a block copolymer consisting essentially of styrene and butadiene, an amorphous alpha olefin polymer, a halide containing flame retardant compound, and an antimony compound.
U.S. Pat. No. 4,229,554 discloses combining an antistatic agent and a flame retardant agent into a thermoplastic resin, but does not mention potential use of the combination in a thermoplastic foam.
U.S. Pat. No. 4,556,680 describes the preparation and use of polystyrene expandable beads having antistatic properties by adding antistatic compounds to the beads during the pre-expansion step. This patent also discloses combining a flame retardant agent with the antistatic agent to make a polystyrene expandable bead that has antistatic and flame retardant properties, but no mention is made of using this technology to make polyethylene foam.
Although the foregoing references indicate that formation of a cellular thermoplastic polymer mass is well known and that numerous practical techniques are available, and further that either an antistatic agent and/or a flame retardant agent can be incorporated into the cellular thermoplastic mass, none of these references recognize or appreciate the advantages which stem from combining an antistatic agent and a flame retardant agent into non-crosslinked or crosslinked thermoplastic polymer foam bead, such as a polyolefin cellular bead, using a single extrusion process. Since it was previously impossible to obtain such non-crosslinked or crosslinked polyolefin foam beads from commercial suppliers, it has been necessary to coat polyolefin foam beads or articles made therefrom with other foams, films, foils and/or liquid or dry coatings.
In many end-use applications, it is desirable to obtain polyolefin foams that will not build up static electricity charges and will not burn. The advantages of this type of foam include safer shipping and safer warehousing or storage of sensitive electronic circuits aboard ships and planes, especially in military craft.
Foams molded from polyolefin beads overcome may of the drawbacks of thermoplastic foams, such as polystyrene foams. Some generally available polyolefin foam beads include non-crosslinked or crosslinked polypropylene or polyethylene. Both of these materials possess greater solvent resistance than polystyrene and are also more resistant to high temperature. Furthermore, polyolefin foam is much more resilient and flexible than polystyrene foam and, therefore, has greater use in the packaging of fragile items. Polyolefin foam also maintains much of its cushioning effect after even many impacts and, therefore, lends itself for use as packaging for long distance transport or reusable packages.
In the case of some thermoplastics, such as polyethylene, a substantially crystalline polymer, the temperature range for good molding of foam beads is quite narrow. If the molding temperature is too low poor fusion will result, the molding will not possess optimum tear resistance, and large voids or unfused pockets could exist in the molding. If the molding temperature is too high, the polyethylene becomes too flowable and the structural integrity of the foam is destroyed, resulting in a collapsed, misshapen molding.
To give the polyethylene a greater resistance to temperature and to widen the temperature range for molding, polyethylene is crosslinked. This allows the polyethylene foam to be molded using steam as the heat source without the polyethylene suffering decomposition.
Moldable crosslinked polyethylene foam beads are presently manufactured in several ways. Polyethylene beads containing a chemical crosslinking agent, such as dicumyl peroxide, and can be suspended in an aqueous solution and heated to the proper temperature to trigger the crosslinking reaction. Polyethylene resin can also be crosslinked by subjecting the particles to high energy radiation, such as X-rays or electron beams. The resultant crosslinked resin particles can then be impregnated with a hydrocarbon or chlorofluorocarbon blowing agents, or the like, such as butane, pentane, dichlorodifluoromethane, etc., by charging an aqueous suspension of the crosslinked polyethylene beads under pressure with the blowing agent. The solution is then heated and stirred in an autoclave to impregnate the beads with the blowing agent. Such processes are described in U.S. Pat. Nos. 4,399,087 and 4,436,840.
Since the blowing agent incorporated in the crosslinked polyethylene particles will readily dissipate, the expandable beads must be stored under pressure or at lower temperatures than ambient or, as is more often the case, immediately pre-expanded. The expansion ratio of these pre-expanded beads is usually between 10 and 45 to 1. Before molding, the beads are usually subjected to a pressurizing step wherein the beads are placed in a container which is charged with pressurized gas, usually air or a chlorofluorocarbon/air mixture. Such processes are described, for example, in U.S. Pat. Nos. 4,399,087 and 4,443,393. This seep raises the pressure of the gas inside the cells of the foam beads to above atmospheric pressure, thus imparting the additional expandability needed during molding. The beads must be molded soon after this step or the additional pressure inside the cells of the beads will be dissipated.
In another method, low density polyethylene resin and a hydrocarbon or chlorofluorocarbon blowing agent are melt mixed and extruded into strands which are cut into beads. These beads are then exposed to high energy radiation to induce crosslinking and to impart to the beads the thermal resistance needed to easily mold the particles. These beads require special molding equipment as no additional expandability is incorporated into the beads prior to molding.
The aforementioned chemical method of crosslinked polyethylene bead manufacture is disadvantageous in that a relatively large and expensive autoclave-type reactor is needed to impregnate the resin with the blowing agent. Furthermore, the method is a batch process wherein a certain quantity of the moldable crosslinked polyethylene beads are manufactured. After a batch of the beads are formed, the entire quantity must be promptly treated and/or stored. This requires large storage facilities. In addition, the beads must be pressure treated prior to molding to impart additional expandability to the foam. This process requires substantial time, as the beads will be destroyed or damaged if the pressurizing step is carried out too quickly. Therefore, large pressure containers are needed to perform the operation economically.
Using the radiation process discussed, the crosslinked beads can be made on a relatively inexpensive extruder equipped with the proper equipment for granulating the foamed extrudate. However, a relatively expensive and cumbersome radiation source is required to crosslink the foam. Furthermore, as it is generally not feasible to perform the crosslinking step at a large number of manufacturing locations, the process generally requires the crosslinking step to be performed at a single, large, central manufacturing facility, or at a handful of such facilities. Furthermore, since high energy radiation does not easily or quickly penetrate into the foamed plastic structure, the degree of crosslinking is often much less on the inside portions of the foamed beads than on the outsides, causing the beads to have deficient thermal resistance.
U.S. Pat. No. 3,413,244 discloses a process for producing cellular polyolefin products in which a particulate unfoamed polyolefin is foamed within a mold and is simultaneously grafted and crosslinked by units of compounds containing two non-conjugated ethylenically-unsaturated double bonds.
International Application No. PCT/F184/00079, International Publication Number WO 85/01944, discloses foamed, silane-crosslinked polyolefin foam cable coverings which are described as relatively hard and rigid and are produced by extruding a mixture containing polyethylene, a silane hydrolyzable with water, a condensing catalyst and a foaming agent such as water.
U.S. Pat. No. 4,333,898 discloses a method for the production of relatively high density foamed polymers (such as polyethylene) in which the polymer is mixed with a silane, which grafts thereto, and which is then extruded to provide a jacket for a cable or the like. A moist, inert gas is injected into the extruder just prior to extrusion to cause the polymer to foam and the silane-grafted polymer to crosslink.
U.S. Pat. No. 4,456,704 discloses a method for producing crosslinked polyethylene foams. The method utilizes a mixture of a polyolefin resin, a blowing agent, and, optionally, a surface active agent. The polyolefin resin contains a crosslinkable ethylene polymer having on the side chains thereof silyl groups which effect crosslinking upon contact with water. The ingredients are mixed, and the mixture is extruded into a low pressure zone where the resulting extrudate (e.g., in sheet form) is allowed to expand. The expanded extrudate is then brought into contact with a silanol condensing catalyst so that the expanded extrudate is crosslinked upon contact with water.
U.S. Pat. No. 4,606,873 discloses a process for making polystyrene beads, but does not mention polyolefins or crosslinking of the polyolefins prior to expansion.
U.S. Pat. No. 4,870,111 discloses a process for making moldable foam beads comprising a silane-crosslinked polyolefin foam. The beads are made by mixing a composition comprising a silane-modified polyolefin (such as a silane-grafted polyethylene) and a silanol condensation catalyst in an extruder to produce a melt, then injecting a blowing agent into the melt at a rate effective to produce a desired foam density in the extrudate. The foamed polyolefin is then extruded and cut to form foam beads, and the beads are exposed to moisture to produce silane crosslinking of the polyolefin foam.
The foregoing references do not disclose, recognize or appreciate the advantages of making a moldable, non-crosslinked or crosslinked thermoplastic polymer foam bead, such as those made from crosslinkable silane grafted polyolefins or chemically crosslinked polyolefins, according to the method and apparatus disclosed in the present application. The foregoing references also do not disclose, recognize or appreciate the advantages of such a method wherein the polyolefins are crosslinked before they are foamed to enhance the processing characteristics of the foam beads and to enhance the properties of the foam and articles made from the polyolefin foam beads. Such advantages include the increase of melt strength, smaller cell diameter, better cushioning characteristics, and higher melting points.
In addition, none of the aforementioned references disclose a method for the manufacture of a moldable non-crosslinked or crosslinked foam bead in which the bead comprises either (1) a non-crosslinked thermoplastic that is foamable; (2) a chemically crosslinked polyolefin, made from a mixture comprising a polyolefin with a chemical crosslinking agent that is placed in an extruder to produce a melt; or (3) a silane-crosslinked polyolefin foam made by mixing a composition comprising a silane-modified polyolefin (such as a silane-grafted polyethylene) and a silanol condensation catalyst in an extruder to produce a melt; injecting a blowing agent into the melt at a rate effective to produce a desired foam density in the extrudate; extruding the melt into a pressurized atmosphere that is sufficient to prevent appreciable expansion of the polyolefin; cutting the melt and thus forming non-foamed beads suspended in a conveying media, such as water; conveying the beads through a zone where they are crosslinked when required; conveying the beads through a zone where the temperature of the beads is regulated to a desired or effective temperature for foaming; and expelling the beads to a lower pressure where they expand to form moldable non-crosslinked or crosslinked foam beads in a continuous manner.
Improved methods of producing moldable beads of foamed thermoplastic polymers, such as polyethylene or polypropylene, are clearly needed that do not require pressure treatment or radiation and that take advantage of the cellular orientation and strength achieved when expanding a polyolefin that is at its ideal extrusion temperature and/or that is already crosslinked.